Casing collar locator



m 1 www HOL l I `June 26, 1951 H, c. PAGAN v 2,558,427

v CASINO COLLAR LOCATOR FiledMay 8, 1946 4 Sheets-Sheet 1 N @y ZA F, 14' f- 113 y 1F77 Z M 1 =-20 yr-.l0 f -12 fr f 4E 7 7 -jf j- 25 y? y j; 43 23 26 g j f 19 J9 ,f 4 Z7 22 t 27 '24 24 f 2s 29 25 2a v 4 INVENTOR.

HERBERT C. FGN

ATTORNEY:

' June 26,1951 y HC1-AGM 2,558,421

' CASING COLLAR LOCATOR Filed May s, 1945 I 4 sheets-sheet 2 'will vI/NTQR m2115121' c'. 2121511111l SEARCH FL@ June 26, 1951 H, C, FAGAN 2,558,427

CASINO COLLAR LOCATOR Filed May 8, 1946 4 Sheets-Sheet 5 /4 g. 6. C I

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IN V EN TOR. HERBERT Cf FGN' A TTORNEYS June 26, 1951 Filed May 8, 1946 H. C. FAGAN CASING COLLAR LOCATOR 4 Sheets-Sheet 4 ATTORNEYS.

Patented June 26, 1951 dragen mult CASING COLLAR LOCATOR Herbert C. Fagan, Houston, Tex., assignor to Schlumberger Well Surveying Corporation, Houston, Tex., a corporation of Delaware Application May 8, 1946, Serial No. 668,298

8 Claims.

The present invention relates to apparatus for locating the joints between adjoining sections of the tubing or casing which usually lines a bore hole drilled into the earth. More particularly, it relates to new and improved apparatus which utilizes magnetic principles for locating joints in a metallic bore hole casing.

Magnetic type casing joint locators are already known. Generally, such apparatus has included magnetic pole pieces maintained in close proximity to the casing, so that significant changes in the reluctance of the magnetic circuit might take place when the pole pieces pass the annular gaps that exist between adjacent ends of casing sections. These changes in the reluctance of the magnetic circuit are usually reflected as changes in the impedance of a winding operatively associated with the magnetic circuit, and they may be observed as changes in the current consumed by the winding if the latter is energized by a constant voltage source.

While magnetic casing joint locators of this type have been operated successfully, they have not been widely used primarily because they are expensive and relatively difcult to operate and maintain. Since they depend for proper operation on maintenance of contact or close proximity with the casing, the wear is great and the rates of ascent and descent are retarded. Moreover. asource of power and rather elaborate indicating circuits must be provided, and at least one additional conductor is required in the supporting cable to transmit power from the surface to the apparatus and to carry the response of the apparatus to the indicating means at the surface. Also, changes in inductance of the supporting cable as it is wound and unwound from the winch causes base line drifts.

The present invention provides a new and improved magnetic type casing collar locator which is free from the deficiencies of the prior art, is extremely sensitive in operation, and is sturdy, cheap and easy to construct. By its use, either cement filled or empty casing joints may be accurately located with equal facility, and it may readily be lowered into the bore hole with other instruments such as a gun perforator and/or switch to check the bottom of the hole without affecting the operation of such auxiliary equipment and Without necessitating a mechanical or electrical redesign of such equipment. Moreover, it is sturdy enough to withstand the shock and concussion produced by a perforating operation without damage, and the indications obtained may be safely Vsent up a conductor in the gun perforator circuit Without any danger of causing the gun perforator to malfunction.

It is an object of the invention to provide a new and improved apparatus for accurately locating the joints between adjacent ends of sections of the casing lining a bore hole drilled into the earth.

Another object of the invention is to provide a magnetic differential type of casing joint locator wherein no physical contact need be maintained between the locator and the casing.

A further object of this invention is to provide a casing collar locator which does not require electrical energy to be furnished either from the surface or from a source lowered into the bore hole.

Still another object of the invention is to provide a casing collar locator of extremely low electrical resistance which may be used in series with other electrically controlled apparatus without materially aecting the characteristics or the operation of such other apparatus.

An additional object of this invention is to provide a casing collar locator which will locate the casing joints irrespective of whether or not they are lled with foreign material such as cement or other non-magnetic material.

A further object of this invention is to provide a new and improved casing joint locator of the above character which is not affected by a permanent or temporary magnetism that may be present in the casing.

Another object of this invention is to provide a casing joint locator wherein a high signal to noise ratio is obtained so that clearly distinguishable signals may be received.

In accordance with the invention, the casing joint locator comprises two approximately similar magnetic circuits, each of which includes a portion of the casing, arranged essentially in the form of a bridge so that small unbalances are easily detected. As long as the two magnetic circuits are similar, no signal is produced, but if either one of the magnetic circuits is altered, as occurs when the device approaches a casing joint, the magnetic lines of flux are forced through a winding having a core of high permeability which produces a signal of suicient strength that it may be sent directly to the'surface Without amplification or alteration in any way. The device utilizes permanent magnets in the magnetic circuit so that no electrical energy need be supplied.

The invention may be better understood from the following detailed description of several rep- 3 resentative embodiments taken in conjunction with the accompanying drawings, in which:

Figure 1A is a schematic diagram showing the basic principles of operation of an improved casing joint locator constructed according to the invention;

Figure 1B shows the casing joint locator of Figure 1A in position at the level of a casing joint in a bore hole;

Figure 2A illustrates schematically another embodiment of the invention which utilizes the principles shown in Figures 1A and 1B;

Figure 2B shows the casing joint locator of Figure 2A in position at the level of a casing joint in a bore hole;

Figure 3 is a view in vertical section through another modication which employs cylindrical magnets in place of bar magnets;

Figure 4 is a typical schematic circuity showing how a casing joint locator constructed according to the invention may be run in conjunction with a perforating or sample taking gun or other apparatus and a switch to check the depth of a bore hole;

Fig. 4A is a partial view in longitudinal section of the casing joint locator and bottom switch employed in the circuit of Fig. 4;

Figure 5 shows idealized records produced by the devices illustrated in Figures 1A, 1B, 2A, 2B and 3;

Figure 6 illustrates a further embodiment of the invention in which magnetic principles are utilized for locating both casing joints and the bottom of a well;

Figure 7 is a schematic diagram of the electrical circuit of the apparatus shown in Figure 6; and

Figure 8 is a representative curve taken in the field with apparatus of the type shown in Figure 6 and illustrating both casing joint and well bottom signals. Y

In Figures 1A and 1B, a casing joint detector rassembly I0 is shown positioned in the steel casing II of a bore hole I2 drilled into the earth. The assembly I0 is adapted to be supported and moved along the bore hole I2 by means of an insulated electrical cable I3 which reaches to the surface of the earth. Signals from the detector assembly I0 are also sent through the cable I3 to the surface of the earth, where they may be exhibited by a suitable indicating device or recorder I4, one terminal of which is grounded at I 6; The detector assembly I0 comprises essentially a pair of similar permanent horseshoe magnets l1 and I8 between which is interposed a high permeability core I9 around which is disposed a suitable winding 20. One end of the winding 29 is connected to the conductor I3 as shown, and the other end may be grounded as at 2| or it may be connected to another conductor (not shown) reaching'to the surface of the earth, as desired.

vThe permanent magnets I1 and I8 are symmetrically disposed about a horizontal center line, so that the magnetic uxes produced thereby ow in opposite directions in the casing II. As shown in Figure 1A, the two south poles of the magnets I1 and I8 may be located adjacent the opposite ends of the magnetic core vI9 although the apparatus will workequally well4 with the north poles of the magnets I1 and I8 adjacent the opposite ends of the core I9. Referring to Figure 1A, the magnetic ilux ows out from the north .pole ofy the magnet 4I1 downwardly through the casing II and back intothe south pole of the magnet I1, the magnetic circuit being completed through the magnet as shown by the dotted lines. The magnetic flux from the magnet I8 flows outwardly from Yits north pole, upwardly through the casing II as shown, and into the south pole of the magnet I8.

When the detector assembly I9 is in a uniform portion of the casing I I, that is, one in which no casing joint is present as shown in Figure 1A, the two magnetic circuits of the magnets I1. and I8 are independent and no resultant magnetic ux flows through the vcore I9. When a casing joint 2| is encountered as the detector assembly is moved upwardly in the bore hole I2, for example, as shown in Figure 1B, the air. gap adjacent the south pole of the permanent magnet I1 is materially increased so that the iiux from the north pole of the magnet I1 travels downwardly through the casing II, through the coupling 22 and into the south pole of the magnet I 8, instead of following the path shown in Figure 1A. lln order to co-mplete the magnetic circuit of the magnet I1, the flux then ows upwardly through the magnetic core I9 and induces a corresponding voltage in the winding 20, which is exhibited by the meter or recorder I4 at the surface of the earth or by any other suitable apparatus.

As the raising of the detector assembly I0 with respect to the casing Il continues, the south pole of the permanent magnet I8 also encounters the casing joint 2|. When this occurs, the reluctance of its magnetic circuit increases and part of the magnetic flux from the north pole of the lower permanent magnet I8 passes upwardly through the casing II and the coupling 22, through the south pole of magnet I1 and downwardly through the magnetic core I9, inducing a Voltage in the opposite direction in the winding 29. A representative curve showing the signals produced at two casing joints at diierent levels in a bore hole is shown in Figure 5. The paths taken by the magnetic flux when the magnet I8 encounters the casing joint .2| will be readily understood and this: position ofthe detector assembly I0 is not illustrated in the drawings.

The modification shown in Figures 2A and 2B of the drawings utilizes essentially the same operating principles shown in Figure 1. However, instead of using horseshoe magnets, permanent bar magnets 23 and 24 are placed between high permeability discs 25 and 26, and 21 and 28, respectively, and the magnetic core I9 as in Figure 1 isv disposed between the high permeability discs 26 and 21. The core I9 is provided with a pickup winding 2U as in Figure 1. The entire assembly is enclosed in a non-magnetic casing 29 preferably made of Bakelite or other mechanically strong material. The ends may be closed in any suitable fashion and attached to the supporting cable I3. Arrangements for attaching bore hole devices to either steel or electric cable are well known in the art and form no part of the present invention. Such connections are therefore shown in schematic form only in the drawings.

Figure 2A shows the modified device ina-section of plain casing while Figure 2B shows the same device in the Vicinity of a casing joint.2l..

It will be noted that this modification uses the entire circumference of the casing I.I as part of the magnetic path instead of one narrow zone as in the case of Figures 1A and 1B.

The operation of. the. device of Figure 2 is analogous to that explained in connection with Figure 1 above. When no casing joint is present as shown in Figure 2A, there is an upper magnetic circuit comprising the magnet 23, the pole piece 25, the air gap between the pole piece 25 and the casing Il, the adjacent section of the casing II, the air gapbetween the casing II and the pole piece 26, and the pole piece 26, as indicated by the dotted lines. The magnetic path of the ilux from the permanent magnet 24 is similar to that of the magnet 23 except that the ilux flows upwardly through the casing II.

When this modification is raised to the level of a casing joint 2|, as illustrated in Figure 2B, the magnetic path for the ux from the magnet 24 is not disturbed. However, the reluctance of the magnetic circuit of the permanent magnet 23 is increased in the vicinity of the pole piece 26 with the result that the flux from the magnet 23 ows downwardly through the casing I I, through the casing collar 22, into the pole piece 21, and then upwardly through the magnetic core I9. This produces a voltage in the winding 20 corresponding to the portion b of the curve shown in Figure 5. As the raising of locator assembly through the casing II continues, the pole piece 21 is moved to a position adjacent the gap 2| between adjacent ends of two sections of the casing l I. This increases the reluctance of the magnetic circuit at this point so that magnetic flux flows down-wardly through the magnetic core I9 and produces a voltage pulse in the opposite direction corresponding to the peak a on the curve shown in Figure 5.

Figure 3 shows a further modication which is preferred for eld operations. This modification operates in essentially the same manner as the device shown in Figures 2A and 2B. The principal difference between the two modifications is that the device shown in Figure 3 uses cylindrical magnets such as loud speaker magnets, which are of very high quality and easily obtained, instead of bar magnets.

Referring now to Figure 3, the apparatus is mounted in a housing comprising a tubular casing 30 made of solid or laminated plastic or other suitable material and enclosed at its top and bottom ends by the closure members 3| and 32, respectively, made of suitable non-magnetic material. The upper closure member 3| is adapted to be secured in the conventional manner to a supporting cable (not shown) by means of which it may be lowered and raised in a. bore hole. Mounted in the upper portion of the casing 3l is an assembly comprising a pair of spaced apart pole pieces 33 and 34 disposed at opposite ends of a cylindrical permanent magnet 35 and secured to one another in any suitable manner by a hollow rod 36, made of suitable non-magnetic material such as brass for example. The pole piece 33 may be threaded or otherwise tightly secured to the top closure member 3|, as shown.

Mounted in the lower portion of the casing 3l is a second assembly comprising the spaced apart pole pieces 31 and 38 disposed at opposite ends of a cylindrical permanent magnet 39 and rigidly secured to one another in any suitable manner by means of a rod 4l made of brass or other suitable non-magnetic material. The pole piece 38 may be threaded or otherwise secured to the bottom closure 32, as shown. Interposed between the pole pieces 34 and 31 and rigidly secured thereto is a rod 46 of high permeability magnetic material, having a Awinding 42 thereon connected to the conductors 43 and 44 which lead through an axial passage 45 formed in the rod 46. the pole piece 34, the rod 36, the pole piece 33 and the top closure 3| to indicating or recording apparatus at the surface of the earth.

The housing 3U may have a number of holes 41 formed therein to admit the bore hole liquid (not shown) if desired. This eliminates the necessity of making the casing 30 strong enough to withstand the high pressures usually encountered in a bore hole. Where this is done, suitable precautions such as, for example, varnishing and baking windings and other parts of the apparatus, should be taken to make such parts waterproof so that they will not be damaged by the presence of the bore hole liquid in the casing 30.

The magnets 35 and 39 are preferably of the type customarily used in permanent magnet type loudspeakers, and they may comprise a series of such magnets stacked on top of one another. Preferably, the outside diameter of the speaker magnets should be slightly less than that of the pole pieces 33, 34, 31 and 38, so that the ow of substantially all of the magnetic flux will be confined to the latter pole pieces.

The thickness of the pole pieces 33 and 38 should preferably be made considerably greater than the maximum width of the gap between the ends of two adjacent sections of casing, so that the reluctances of the magnetic circuits will not be changed appreciably when the pole pieces 33 and 38 pass a easing joint. On the other hand, the thickness of the intermediate pole pieces 34 and 31 should preferably be made less than the width of the casing joint gap so that significant changes in the reluctances of the magnetic circuits will occur when the pole pieces 34 and 31 pass a casing joint, and clearly distinguishable signals will be received by the recorder I4 (Figure 1A) The manner of operation of the modification shown in Figure 3 will be readily apparent from the preceding description and from the operational description of the modification illustrated in Figures 2A and 2B.

The casing joint locator may be used alone or in conjunction with any other piece or pieces of apparatus. A representative combination of a casing joint locator constructed according to the invention and other bore hole apparatus is shown schematically in Figure 4. In Figure 4, the winding 42 of the casing joint locator is connected in series with the series-connected gun igniters 48, as described in Patent No. 2,141,827, which may in turn be connected in series with a bottom switch 49, of the type shown in Schlumberger Patent No. 2,361,064. As shown in Fig. 4A, the bottom switch 49 may comprise, for example, a plurality of fixed contacts 60 and a movable contact 6I on an actuator rod 62. The rod 62 terminates at its lower end in a disc-like actuator 63. The switch 49 is normally maintained by a spring 64 and is adapted to be opened when the actuator 63 engages the bottom of the well. The movable switch contact 6| is grounded to the case of the instrument, the earth serving to complete the electrical circuit. For this purpose, it may be convenient to run the wire 44 (Figure 4A) through an axial passage 65 in the lower portion of the apparatus in order to avoid running conductors along the outside of the apparatus. The wire 44 is connected to the fixed switch contacts 60 of the bottom switch 49. If it is not desired to cause the circuit to be completed through the earth, a second conductor in the cable or the cable sheath may obviously be used for this purpose.

y! In operation, a pair of ganged switches 50 and 5| may be provided so that` in one position, the circuit is completed through the indicator or re. corder I4. In this position, the depth of the plug may be checked by the bottom switch, using a small series battery if necessary, and the dis tance between the bottom of the hole and a casing joint thus determined. Whenit is desired to actuate the gun perforator or other apparatus, the ganged switches 56 and 5I are Vthrown tothe upper position so that electrical energy may be sent from the surface to fire. the gun igniters selectively in the well known manner.

In Figure 6 is shown a modification of theapparatus shown in Figure 3in which magnetic principles are usedto provide an indication when theapparatus reaches or leaves they bottom of a Well. In this embodiment, the casing 30 is extended beyond the lowermost pole piece 38 and is provided with a closure member 52, forming a chamber 53 at thebottom of the apparatus. The closure member 52 has an axial bore 54 therein through whichA passes an actuator rod 55 having a magnet 56 secured at the upper end thereof and terminating at its lower end in a disc-like actua.- tor 5l.`v The actuator 5l is normally maintained away from the closure member 52 by means of a compression spring`58 which is interposed therebetween,and whose strength is insufficient to support the weight of the apparatus. One end ofthe pickup coil '42 is connected to the cable conductor .43 and the other end is grounded to the casing at 2|', although two conductors may be used as shown in Figure 3, if desired.

If the field of the magnet 56 is made strong enough, movement of the actuator 5l upwardly or downwardly willcause a sufficient change in flux from the magnet 56 linking the coil 42 to produce a clearly distinguishable signalon the meter I4 at the surface of the earth. Thus, if the apparatus is lifted from a position of rest at the bottom of the well, movement of th-e magnet 56 away from the lower pole piece 38 will produce a signal similar to that shown at c in Figure 8. Movement of the magnet 55 toward the lowermost pole piece 38 will produce a signal ofopposite polarity.

The` signal produced at the bottom of the well can be readily distinguished from signals produced by a casing joint, since the former is of single polarity whereas the latter are of double polarity. Preferably, the field strength of the magnet 56 should be made large enough to make the magnitude of the well bottom signal greater thanf the casing joint signals.

Casing joint locators constructed according to the invention have been built and have proved to be very satisfactory in operations conducted in the field. In the apparatus used, the solenoid winding 42 in Figures 3 vand 6 consists of 1000 turns of No. 16 gauge copper wireon a 11/2 diameter core. The sharpness with which the casing joints arelocated may be controlled by varying the distance between the pole pieces 34 and 31. When fast galvanometers are used in the recorder I4 and the device is raised through the bore ho`e at speeds from 1000 to 6000' per hour, a 6 spacing between the pole pieces 34 and 37 has been found entirely suitable. In practice, the magnets 35 and 33 may be Alnico #5 speaker magnets. The coil 42 may have a resistance aslow as 2 ohms, and a high urrent carrying capacity so that it may be conveniently inserted in series with other bore hole equipment without necessitating a major redesign .of any other auxiliary equipment. v i

From the foregoing, it will be apparent that the invention provides a highly satisfactory device; for detecting the joints between adjacent sections of the` casing lining a bore hole drilled into the earth, either .on the ascent or on the descent. By employing two essentially independent magnetic circuits `Adiierentially responsive primarily to the change in the reluctance produced by the casing joint gap," the device is not materially influenced by the width of the gap between itself and the' inside wall of the casing. In iact, a 3% outside diameter casing joint locator'has beenused successfully to locate accurately the casing joints in an 8 casing without using. any guides or other means for maintaining the device in a fixed position with respect to the casing.. Under such conditions, the signal produced at the level of a casing joint wasfound to be never less than four times the maximum background signal,l

Since no guides or analogous means are required, and since only simple and rugged parts are employed, the casing joint locator of thepresent invention is not adversely affected by vibration or shock such as is produced by the firing of a gun perforator. Further, since the deviceA comprises principally standardv parts such as magnets and pole pieces and does not include ,any elaborate joints, connections or pressure seals, it is simple and cheap to construct and easy to maintain.

It will be apparent that the novel casing joint locator described herein is not adversely affected by thepresence of` any magnetism in the casing. In general, the magnetic properties of the casing do not change materially over the short distance corresponding to the length of thev casing joint locator. Hence both magnetic circuits are influenced in about the same way by magnetism in the casing, and no significant change in magnetic ux is produced in the core of the pickup winding which connects the two magnetic circuits.

Although the foregoing description is directed to the location of casing joints, it will be under# stood that the novel casing joint locator disclosed herein can be used equally well to locate any marked and localized magnetic anomaly in cased or uncased bore holes. For example, it may be used to detect magnetic markers of the type disclosed in the aforementioned copending application of Henri-Georges Doll, Serial No. 443,300, led May 16, 1942, now Patent No. 2,476,137, issued July 12, 1949. In fact, it has been used successfully to detect magnetic marker bullets which were embedded in the formation as far as 12 from the casing.

Although it is advantageous to use permanent magnets in the apparatus described herein, ob-

viously, electromagnets energized from a suitable source located either in the apparatus or at the surface of the earth may be used, if desired. Also, it is not necessary that the field strengths of the two magnetic circuits comprising the casing yjoint locating apparatus be the same, The apparatus will operate equally well with one magnetic circuit stronger than the other; however, one of the peaks a, and b in Figures 5 and '7 will appear larger than the other Where this condition exists.

While several representative embodiments have been described above, the invention is not intendedto be limited thereto, but is susceptible of' numerous changes in form and detail within the scope ofthe appended claims, v -v sies-rte ROOM 9 I claim:

' 1. Detecting apparatus for use in a bore hole, comprising magnet means providing a magnetomotive force of substantially constant magnitude and polarity and adapted to be moved through a bore hole, a pair of longitudinally spaced apart pole means on said magnet means for emittingv and receiving magnetic flux at longitudinally spaced apart locations in the bore hole, third pole means longitudinally spaced apart from said pair of pole means, means forming a magnetic flux path between said third pole means and said magnet means, winding means operatively associated with said magnetic flux path forming means for providing a voltage related to changes in magnetic flux in said flux path, and means for recording said voltage as a function of the depth of vsaid magnet means in the bore hole.

2. Detecting apparatus for use in a bore hole, comprising a magnet providing a magnetomotive force of substantially constant magnitude and polarity adapted to be moved through a bore hole for emitting and receiving magnetic flux at rst longitudinally spaced apart locations in the bore hole, a second magnet providing a magnetomotive force of substantially constant magnitude and polarity spaced apart from said first magnet and movable therewith for emitting and receiving magnetic flux at second longitudinally spaced apart locations in the bore hole, a member of magnetic material connecting said rst and second magnets, means providing a voltage related to changes in magnetic flux in said member, and means for recording said voltage as a function of the depth of said apparatus in the bore hole.

3. In apparatus for detecting joints between sections of casing lining a bore hole, the combination of a housing adapted to be lowered into a bore hole, a pair of longitudinally spaced apart magnets in the housing, each of said magnets providing a magnetomotive force of substantially constant magnitude and polarity and having magnetic flux emitting and receiving portions facing said casing, a member of magnetic material connecting said magnets, a pickup winding operatively associated with said member and means -for recording the output of said pickup winding as a function of the depth of said magnets in the casing.

4. In apparatus for detecting joints between adjoining sections of casing lining a bore hole, the combination of a housing adapted to be lowered into a bore hole on a supporting cable, a magnet providing a magnetomotive force of substantially constant magnitude and polarity mounted in the upper portion of said housing, pole pieces mounted at opposite ends of said magnet and having ux emitting and receiving portions facing the bore hole casing, a second magnet providing a magnetomotive force of substantially constant magnitude and polarity mounted in the lower portion of the housing, pole pieces mounted at opposite ends of said second magnet and having flux emitting and receiving portions facing the bore hole casing, a member of magnetic material connecting the upper pole piece of said second magnet and the lower pole piece of said first magnet, a pickup winding operatively associated with said member, means for recording the output of said pickup winding as a function of the depth of the housing in the bore hole, and a circuit including at least one conductor in said supporting cable for con- 10 necting said recording means to said pickup winding.

5. In apparatus for detecting joints between adjoining sections of casing lining a bore hole, the combination of a housing of non-magnetic material adapted to be lowered into a bore hole on a supporting cable, four longitudinally spaced apart pole pieces mounted in the housing and having ring-like magnetic ux emitting or receiving surfaces facing the bore hole casing, magnets providing magnetomotive forces of substantially constant intensities and polarities operatively mounted between the two upper and two lower pole pieces, a member of magnetic material providing a magnetic path between the two intermediate pole pieces, a pickup winding on said member, recording means at the surface of the earth for recording the output of said pickup winding as a function of the depth of said nonmagnetic housing in the bore hole, and an electrical circuit including at least one conductor in said supporting` cable connecting said pickup winding and said recording means.

6. In apparatus for detecting joints between adjoining sections of casing lining a bore hole, the combination of a housing of non-magnetic material adapted to be lowered into a bore hole on a supporting cable, four longitudinally spaced apart pole pieces mounted in the housing and having ring-like magnetic flux emitting or receiving surfaces facing the bore hole casing, the lengths of the flux emitting or receiving surfaces of the topmost and bottommost pole pieces being greater than the length of the gap between adjoining bore hole casing sections, and the lengths of the ux emitting or receiving surfaces of the two intermediate pole pieces being less than the length of said gap.' magnets of substantially constant magnetomotive forces operatively mounted between the two upper and two lower pole pieces, such that the topmost and bottommost pole pieces are of the same polarity, and the intermediate pole pieces are both of a different polarity, a member of magnetic material providing a magnetic path between the two intermediate pole pieces, a pickup winding on said member, recording means at the surface of the earth for recording the output of said pickup winding as a function of the depth of said housing in the bore hole, and an electrical circuit including at least one conductor in said supporting cable connecting said pickup winding and said recording means.

'7. In apparatus for use in a bore hole, the combination of a housing adapted to be lowered into a bore hole on a supporting cable, a magnet providing a magnetomotive force of substantially constant magnitude and intensity in the housing for emitting and receiving magnetic flux at rst longitudinally spaced apart locations in the bore hole, a second magnet providing a magnetomotive force of substantially constant magnitude and intensity in the housing, spaced apart from said rst magnet for emitting and receiving magnetic ux at second longitudinally spaced apart locations in the bore hole, a member of magnetic material providing a magnetic path between said rst and second magnets, a pickup winding operatively associated with said member, a plurality, of electrical gun ignitersl in the housing, an elec-I `trical circuit including at least one conductorf` in said supporting cable connecting said pickup a winding and gun igniters in series, recording means at the surface of the earth for recording the -output of said pickup winding as a function 0f the depth of said housing in the bore hole, a

8. In apparatus for use in a bore hole, they combination of a housing adapted to be lowered into a bore hole on a supporting cable, a magnet of substantially constant magnetomotive force in the housing for emitting and receiving magnetic ux at rst longitudinally spaced apart locations in the bore hole, a second magnet of substantially constant magnetomotive force in the housing, spaced apart from said first magnet for emitting and receiving magnetic ux at second longitudinally spaced apart locations in the bore hole, a member of magnetic material providing a magnetic path between said first and second magnets, a pickup winding operatively associated with said member, recording means at the surface of the earth for orrfhg pyfnutmof. said; 919311.? windings'a function of lthe depth of said housing in the bore hole, an electrical circuit connecting said pickup coil and recording means, a magnetic member disposed below said second magnet, and means retaining said magnetic member yieldingly in predetermined relation to said second magnet and permitting relative movement of said second magnet and magnetic member when the apparatus is set on, or raised from the bottom of the bore hole.

HERBERT C. FAGAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 5, Number Name Date 824,812 Plimpton July 3, 1906 1,823,810 Wall Sept. 15, 1931 1,992,100 Stein Feb. 19, 1935 2,098,064 Pfaffenberger Nov. 2, 1937 20 2,104,643 Greenslade Jan. 4, 1938 2,228,623 `Ennis Jan. 14, 1941 2,246,542 Smith June 24, 1941 2,309,835 Fearon Feb. 2, 1943 2,338,991

Arnold Jan. 1l, 1944 

